Not Applicable
Not Applicable
1. Field of the Invention
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 AND 1.98
In the competitive field of radio telephones, product development is influenced the most by the quest to minimize their manufacturing costs, size and current consumption. In future design of radio telephones, work will be focused particularly on developing new system designs for a radio frequency module, because the rf-module takes up approximately one half of the area of the printed circuit board of a telephone and accounts for a large portion of the manufacturing costs.
The simplest transmitter of the prior art can be implemented by means of a transmission frequency synthesizer. In using a transmission frequency synthesizer, modulation is added directly to the transmission frequency, whereupon the transmitter, as shown in FIG. 1, includes only a synthesizer 11, a modulator 12, and possibly a bandpass filter 13 and a power amplifier 14. Two frequency components separated by 90 degrees phase shift and required by the quadrature modulator 12, are usually produced from the transmission frequency transmission signal f1 of the synthesizer 11 by means of a divide-by-2 divider 15, providing good phase accuracy. Therefore, the signal also must be multiplied by two by means of a multiplier 16 before it enters the divider 15 to preserve the transmission frequency. The bandpass filter 13 is often needed because of noise caused by the modulator 12, even though the transmission frequency fTX is produced without intermediate frequencies. Said construction offers a notable savings in components compared to a transmitter based on an intermediate frequency, and thereby, in principle, it is possible to achieve the goals in manufacturing costs, size and current consumption mentioned above. The savings in components are mainly based on the fact that the transmitter does not require a mixer stage as conventional heterodyne constructions do, nor offset oscillators that produce local frequencies.
However, there are several problems associated with a transmitter construction based on a transmission frequency synthesizer. In digital telephones based on time-division technology, transmission is allowed only during a transmission time period, resulting in burst-like transmission. It is known that burst-like transmission causes strong electromagnetic disturbances in the surroundings. The disturbances are particularly detrimental to a synthesizer operating at the same frequency as the transmission, because said disturbances are easily coupled to the voltage-controlled oscillator of the synthesizer, either electromagnetically or along a signal path, due to insufficient isolation. In continuously operating analog radio telephones, in which transmission and reception occur simultaneously along different channels, the problem is that modulation added to the transmission frequency is coupled to the voltage-controlled oscillator of the transmission frequency synthesizer, which advantageously also produces the local frequency of the receiver. The error caused in the local frequency of the receiver by the modulation of the transmission becomes mixed to the intermediate frequency in the receiver, causing residual modulation in the reception signal, thereby disturbing its detection. A similar problem is also possible in a continuously operating CDMA system.
Electromagnetically coupled disturbances can be minimized with good rf shielding, which, in this case, should be directed to not only the case of the radio telephone, but to the entire frequency synthesizer construction. However, rf shielding consumes space on the printed circuit board and complicates manufacture of the device to the extent that space and cost savings realized by implementing a transmission frequency synthesizer are at least partly lost.
The case of a radio telephone is usually designed to protect the device from external electromagnetic disturbances. If the transmitter is based on a synthesizer operating at transmission frequency, product development and production testing of at least the transmission frequency synthesizer must be performed with the case in place to minimize the effects on the synthesizer of disturbances originating in the transmitter of the device itself. Naturally, this hinders efficient testing.
The object of the present invention is to provide a simple frequency synthesizer construction by which means the problems described above caused by coupling of electromagnetic disturbances are at least partly eliminated. By applying the present invention, it is possible to manufacture a radio telephone with low manufacturing costs, small size and low current consumption. The present invention is characterized by what is presented in the characterizing part of claims 1 and 5.
The output frequency of the frequency synthesizer of the present invention is a multiple of the transmission frequency, which is divided before modulation to produce the transmission frequency. In an advantageous embodiment, the frequency synthesizer produces a frequency that is twice the transmission frequency, which is divided by means of a divide-by-2 divider to produce the frequency components separated by 90 degrees phase shift and required by the quadrature modulator. The modulated signal, which in this case suitably is of transmission frequency, is filtered conventionally with a bandpass filter, eliminating all signal components except those belonging to the transmission channel, thereby preventing power amplification of the frequency produced by the synthesizer, which is twice the transmission frequency, and thereby preventing the disturbances described above.
The present invention is described in detail below, with references to the enclosed drawings.